Light generating or lighting devices using LEDs are known in the art and are being used nowadays for a variety of lighting applications. Amongst others, such devices are used in retrofit applications, e. g. to replace common incandescent lamps. In the latter case, it is often required to drive the retrofit light generating device with a variable voltage, e. g. mains or AC voltage. Since LEDs typically cannot be driven directly with mains voltage, a power supply circuit is required.
To provide a relatively inexpensive setup for driving a LED light generating device with mains voltage, it is known to connect a plurality of LEDs in series, so that the overall voltage drop of the thus formed LED string corresponds to the mains voltage. However, due to the inherent alternating behavior of mains voltage, such an LED string cannot be operated during all phases of the AC waveform, i. e. light is not generated when the provided voltage drops below the overall forward voltage of the LED string. Accordingly such circuits often exhibit optical flicker and a relatively low current flow angle.
A further problem may occur when a LED light generating device is intended for use with different mains voltage level systems, such as a 230V mains system and a 120V mains system.
If a device designed for 230V mains is operated with 120V, the voltage will be significantly lower than the predefined overall forward voltage during the largest part of the waveform, leading to a visible flicker and a poor brightness.
If, on the other hand, a device designed for 120V is operated with 230V mains voltage, the voltage will be too high during large parts of the waveform and, although flicker may in fact be reduced, the surplus voltage usually is handled by a linear regulator or bleeder circuit, which results in large amounts of energy being dissipated. In either case, operation of the device with the “wrong” mains voltage is very inefficient.
A solution to the first problem is disclosed in WO 2010/013172 A1, which discloses an illumination device comprising multiple LEDs. According to the document, a plurality of LEDs is provided and connected with a switch matrix. The switch matrix is connected with a controller, so that the LEDs may be connected in multiple different control states, which are adapted for different voltage levels. The switch matrix allows to connect all LEDs mutually in series and in parallel. In a further control state, the LED groups are connected, so that two of said LEDs are connected in parallel and at least two of said LEDs are mutually connected in series. The controller is provided to set the control state of the switch matrix according to a signal, indicating the momentary value of the supplied AC voltage.
The setup of WO 2010/013172 A1 thus allows to adapt the configuration of the device according to the momentary AC voltage. Although selected, i.e. so-called “binned” LEDs are used, the process of selection or “binning” increases the overall manufacturing costs noticeably. Therefore, it is an object of the present invention to provide an enhanced light generating device.